Electric motors are becoming increasingly sophisticated and modern designs are capable of ever higher rotational speeds. One such design of electric motor is a brushless DC motor as used by the applicant in its range of handheld vacuum cleaners and which is capable of being driven at rotational speeds in the region of 100000 rpm.
The use of high speed motors in fluid-moving applications such as vacuum cleaners, fans and hair care appliances is technically attractive since a fan unit equipped with a physically small motor can nonetheless produce very high fluid flow rates.
However, all motors produce vibration as they spin and in such high speed motors it is a particular technical challenge to reduce the noise that is generated by the motor and, therefore, transmitted to its immediate environment. This is particularly the case with brushless DC motors which tend to generate high frequency tones by virtue of the phase excitation necessary in such a motor.